The present invention relates to the protection of a surge arrester. More particularly, the present invention relates to a surge arrester protection system and method which, among other things, reduces the production of molten metal and provides a trap to catch any molten metal that may be generated during the venting process.
A surge arrester, also called a lightning arrester, is commonly connected in parallel with a comparatively expensive piece of electrical equipment in order to shunt over voltage surges, such as those caused by lightning strikes, to ground, thereby protecting the equipment and circuit from damage or destruction. A modern surge arrester typically includes an elongated enclosure made of an electrically insulating material, a series of voltage dependent nonlinear resistive elements retained within the housing, and a pair of electrical terminals at opposite ends of the housing for connecting the arrester between line and ground.
The voltage dependent nonlinear resistive elements employed are typically metal oxide varistor elements formed into relatively short cylindrical disks which are stacked on top of each other within the enclosure. Other shapes and configurations are also used for the varistor elements. The varistor elements provide either a high or low impedance current path between the arrester terminals depending on the voltage appearing across the varistor elements themselves. More specifically, at the power system's steady state or normal operating voltage, the varistor elements have a relatively high impedance. As the applied voltage is increased, gradually or abruptly, their impedance progressively decreases until the voltage appearing across the varistors reaches the elements' breakdown voltage, at which point their impedance dramatically decreases and the varistor elements again become highly conductive.
Accordingly, if the arrester is subjected to an abnormally high transient over voltage, such as resulting from a lightning strike or power frequency over voltage, the varistor elements become highly conductive. In this highly conductive state, the varistor elements conduct the resulting transient current to ground. As the transient over voltage and resulting current dissipates, the varistor elements' impedance once again increases, restoring the arrester and electrical system to their normal, steady-state condition.
Occasionally, the transient condition may cause some degree of damage to one or more of the varistor elements. Damage of sufficient severity can result in arcing from one terminal to the other within the arrester enclosure, leading to extreme heat generation and gas evolution as the internal components in contact with the arc are vaporized. The gas evolution causes the pressure within the arrester to increase rapidly until it is relieved by either a pressure relief mechanism or by the rupture of the arrester enclosure. The failure mode of arresters under such conditions may include the expulsion of components or component fragments in all directions. Such failures pose potential risks to personnel and equipment in the vicinity. Equipment may be especially at risk when the arrester is housed within the equipment it is meant to protect, e.g., in the tank of a transformer.
Attempts have been made to design and construct arresters which will not catastrophically fail with the expulsion of components or component fragments. One such arrester is described in U.S. Pat. No. 4,404,614, the contents of which are incorporated herein by reference in its entirety. U.S. Pat. No. 4,404,614 discloses an arrester having a non-fragmenting liner and outer housing, and a pressure relief diaphragm located at its lower end.
Despite such attempts, the above-described arresters may still fail with expulsion of components or fragments of components. This may in part be due to the fact that when the internal components in these arresters fail, the resulting arc vaporizes the components and generates gas at a rate that cannot be vented quickly enough to prevent rupture of the arrester enclosure.
Solutions to the above-mentioned problem have been made which force the arc to form outside of the arrester. For example, FIG. 1A illustrates one possible solution to the above-mentioned problem as described in U.S. Pat. No. 4,930,039, the contents of which are incorporated here by reference in its entirety.
The arrester of FIG. 1A, includes a subassembly enclosure, one or more electrical components stacked in series within the enclosure, and outlets formed in the wall of the enclosure for transferring an internal arc outside a length of the enclosure and diverting the arc current around some, or all, of the internal components. The outlets allow the ionized gas which is formed during failure, to be vented through the wall of the enclosure thereby forming an alternate conducting path in parallel with the higher impedance path formed by the internal components.
Another approach used to keep the arc produced during the venting process outside a length of the arrester is that made by the use of a monolithic, active resistor core made of voltage-dependent resistance material based on zinc oxide. For example, FIG. 1B illustrates a surge arrester with a monolithic core based on zinc oxide as described in U.S. Pat. No. 4,729,053, the contents of which are incorporated herein by reference in its entirety. The resistor core is sealed in a insulator jacket which is made as a cast-around mass in epoxy resin, concrete polymer, silicone resin or as a sheathing in the form of a shrink-fit tube, a coating, a paint or a glazing.
The arcs produced upon the failure of arresters similar to the ones shown in FIG. 1A and FIG. 1B, typically produce an arc which spans the surge arrester from its two terminals and across the body of the surge arrester. During venting, due to the extreme temperature and current of the arc, components of the surge arrester and its terminals tend to melt, which in turn produces molten metal which can fall to the ground and start a fire and/or harm people or objects nearby.
Therefore, there is a need for a surge arrester which will upon venting produce less molten material, as well as prevent any molten material formed from falling to the ground or escaping the vicinity of the surge arrester.